Pressure transducer

ABSTRACT

A pressure transducer with a diaphragm coupler at one end and a sensing device at the other end connected by a capillary tube. The sensing device includes a cap having a deformable surface to which a strain gauge arrangement is attached, and defining, with an upper portion of the transducer, a thin sensor compartment whereby a liquid filling the capillary tube and sensor compartment will transmit a pressure exerted against the diaphragm coupler to the deformable surface of the cap.

United States Patent Eggleston etal.

[54] PRESSURE TRANSDUCER [72] Inventors: Gerard Eggleston, Danvers;Bosworth,

George, Holbrook, both of Mass. I

[73] Assignee: Mlcrodot, lnc., Westwood, Mass.

[22] Filed: July 28, 1970 [21] Appl. No.: 58,817

52 us. CI "73/393, 73/398 AR, 338/4 51 ..c01| 9/04 581 Field of Search..73/39s R, 298 AR, 406, 393-,

[56] References Cited UNlTED STATES PATENTS 3,349,623 10/1967 Pastan..73/398 AR 1 July 25, 1972 Primary Examiner-Donald O. Woodie]Attorney-Wolf, Greenfield & Sacks [5 7] ABSTRACT A pressure transducerwith a diaphragm coupler at one end and a sensing device at the otherend connected by a capillary tube. The sensing device includes a caphaving a deformable surface to which a strain gauge arrangement isattached, and defining, with an upper portion of the transducer, a thinsensor compartment whereby a liquid filling the capillary tube andsensor compartment will transmit a pressure exerted against thediaphragm coupler to the deformable surface of the cap.

7 18 Claims, 3 Drawing Figures Patented July 25, 1972 HG. I

FIG. 3

PRESSURE TRANSDUCER BACKGROUND OF THE INVENTION The present inventionrelates to an improved pressure transducer and is more particularlyconcerned with an improved fluid filled pressure transducer such asdisclosed in U.S. Pat. No. 3,349,623 to Pastan, issued Oct. 3 l, 1967.

As discussed in the Pastan patent, fluid filled pressure transducers aredesigned to be used in those systems in which it is undesirable for themedium whose pressure is being measured to enter into the instrument. Insuch cases, the instrument itself is filled with a fluid which iscoupled by means of a diaphragm or some other device to the medium whosepressure is to be measured, and the fluid which fills the instrumentdirectly transmits the pressure of the medium to the sensing device. Thefluid fill must necessarily have a boiling point higher than thetemperature of the medium to which the instrument is exposed, as theintroduction of vapor pressure into the gauge will produce extraneousreadings at the sensing device which are not representative of thepressure being measured, Similarly, the freezing point of the fluidshould be lower than the minimum operating temperature.

One of the primary objects of the Pastan invention was to prove a fluidfilled pressure transducer providing full scale deflection of thesensing device with a minimum volumetric displacement. The minimumvolumetric displacement of the fluid filling the instrument minimizedthe effect upon the fluid medium whose pressure was being sensed. Theminimum volumetric displacement resulted in a minimum couplerdeflection, and when the coupler was a diaphragm, minimum deflectionmaintained the diaphragm deflection in the linear range. Diaphragmdeflection did not exceed 3 percent of the diaphragm diameter tomaintain linearity.

This prior art device did, however, have some disadvantages associatedwith it. For example, it did not operate efiectively at lower pressures.Also, the device was relatively costly to manufacture.

Accordingly, one important object of the present invention is to providea fluid filled pressure transducer having a wide pressure operatingrange.

It is another object of the present invention to provide a pressuretransducer which is capable of operating at relatively low pressures.According to one aspect of the invention, a strain gauge bridge is usedhaving four active arms and thus provides the same output for lowerpressures as many prior art devices wherein only two bridge arms areactive.

A further object of the present invention is to provide a pressuretransducer that may be designed to have a higher overload capacity thansome known devices. For some applications it may be desirable to providea higher overload capacity instead of providing the capability ofmeasuring low pressures. As previously mentioned, with the device ofthis invention the four active arms of the bridge provide the sameoutput for lower stress levels. Thus the sensing device may be madeheavier so that the same output is provided for the same stress level.The heavier sensing device extends the upper operating range of thedevice thereby providing a higher overload capability.

Still a further object of the present invention is to provide a fluidfilled pressure transducer that may be fabricated inexpensively.

Still a further object of the invention is to provide a transducer whichincludes a pressure sensing device that may be easily removed from therest of the instrument, thereby providing the capability of using thedevice over different pressure ranges.

SUMMARY OF THE INVENTION To accomplish these and other objects, thepressure transducer of this invention includes an elongated frame and acapillary tube extending through the frame and terminating at one endadjacent one end of the frame. A coupler closes that end of the frameand defines with the frame a chamber that is in communication with thecapillary tube. The other end of the capillary tube communicates with asensing device which has a very small deflection throughout its fulloperative range. The sensing device comprises a deformable cap memberhaving a recess therein defining with a portion of the frame a thinsensor compartment in communication with the other end of the capillarytube. A liquid fills the thin sensor compartment, chamber and capillarytube so as to transmit directly the pressure applied against the couplerto the sensing device to render a pressure measuremen In accordance withone embodiment of this invention, a temperature sensing winding isdisposed adjacent to the coupler, which forms part of a separatetemperature compensating system to balance the effect upon the sensingdevice of expansion of the liquid in the instrument.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view ofone form of fluid filled pressure transducer constructed in accordancewith this invention;

FIG. 2 is a plan view of the sensing device of FIG. 1 showing the straingauge arrangement; and

FIG. 3 is a schematic diagram illustrating the strain gauge circuitemployed in the transducer shown in FIG. 1.

DETAILED DESCRIPTION The embodiment of this invention shown in FIG. 1includes a main frame 10, a sensing device 12 provided at the top end ofthe frame, a capillary tube 14 which extends through the frame, and adiaphragm coupler 16 secured to and closing the bottom end of the frame.

The frame 10 includes an upper generally cylindrical portion 22, anintermediate portion 34, a hexagonal portion 60, and stem portion 66.When the transducer is assembled these various portions are suitablyattached to each other such as by welding to provide a tight, sealeddevice.

Upper portion 22 includes a smaller diameter cylindrical top 23 which isthreaded at 24 to mate with the threads 54 of cap 50. The upper surface55 of cylindrical top 23 is machined as is the surface 51 of the recess52 in cap 50, to define a thin sensor compartment 25 which may be filledwith a liquid 40 such as mercury. Upper portion 22 also has acylindrical chamber 27 partially filled with an epoxy 32 to holdcapillary tube 14 and filler tube 15 in place. Filler tube 15 is curvedat its bottom section and extends through opening 28 in the side ofportion 22. The top portions of capillary tube 14 and filler tube 15extend through passages 29 and 31 respectively in portion 22 andterminate at the upper surface of cylindrical top 23. The other end ofcapillary tube 14 terminates adjacent the coupler 16 whereas theopposite end of filler tube 15 terminated just outside passage 28 andmay be provided with a plug 30. The liquid that fills the pressuretransducer may be forced under pressure through filler tube 15, and plug30 which is preferably threaded into tube 15 retains the liquid in thedevice.

Annular welding recess 56 is provided at the bottom end of portion 22and a similar recess 57 is provided at the top end of portion 34. Anannular weld may be formed between these annular recesses to attachframe portions 22 and 34 together. Similarly, other annular recesses areshown in FIG. I for securing the other portions of the device together.

Intermediate portion 34 includes a passage 42 for accommodatingcapillary tube 14 and also includes an annular flange 36 having acircumferentially extending slot 38. Slot 38 is provided so that ahousing member (not shown) may be attached to the frame and cover thesensing device 12 and protect the top portion of the pressuretransducer. The bottom end of intermediate portion 34 includes adownwardly extending flange 46 which meets with the hexagonal portiondisposed below portion 34. Portions 34 and 60 are conventionally weldedat the recesses 56 and 57 provided near their adjacent ends.

Hexagonal portion 60 is provided to facilitate the use of a wrench tomount the transducer in place. It will be noted that mounting threads 70are shown near the bottom of stem 66 which are screwed into a socket inthe machine on which the transducer is used. The capillary tube 14passes through passage 62 in hexagonal portion 60 and further extendsthrough a passage 68 in the stem portion 66.

The diaphragm coupler 16 is located at the bottom end of stem portion22, is relatively flexible, and has a low spring rate. It is subject tominimum stresses as it is completely supported on the inside by thefluid 40 which fills chambers 72 behind it. Unlike diaphragms which areconnected to and work against push-rods, strain tubes or other similardevices in unfilled instruments, which diaphragms operate at high stressand are subject to diaphragm rupture, the diaphragm of this inventionoperates at very low stress levels because it is supported on the backby substantially the same pressure which is exerted against the outsideor exposed face and is thus subjected to only minimum deflections. Thefluid 40 which fills the chamber 72 also fills the capillary tube 14,and compartment 25 which forms part of sensing device 12.

When the transducer is used in a temperature range having a maximum ofabout 750 F, mercury is the ideal fluid to fill the device. Mercury hasa low compressibility and a boiling point in excess of 700 F, so thatwithin that operative range no. vapor pressures will be generated tocause secondary expansion of the chamber 25 to distort the wall 53 ofcap 50.

In a preferred form of the invention, the capillary tube 14 has an innerdiameter of approximately 0.010 inch, and the thin compartment 25defined by cap 50 and cylindrical top 23, is 0.005 inch. The chamber 72is approximately 0.010 inch deep measured from the inner surface of thediaphragm l6, and the thickness of wall 53 is between 0.015 and 0.030inch.

The sensing device of the present invention includes the cap 50 havingrecess 52 defining wall 53. Compartment 25 can be very accurately formedin the embodiment of FIG. 1 with the use of conventional machiningtechniques. By accurately machining recess 52 to the correct depth andalso machining the upper surface of cylindrical top 23 of portion 22, avery thin compartment 25 is achieved and deformable wall 53 is definedwithin close tolerances. (In FIG. 1 wall 53 is exaggerated in size, asin compartment 25.) This degree of accuracy using relatively simplemachining techniques was not readily obtainable in prior art devices.

In FIG. 1 it will be noted that temperature compensation means is alsoprovided. This feature is particularly desirable when the transducer isdesigned to operate over a wide temperature range and the diaphragmcoupler is unable to wholly dissipate the effects of liquid expansion inthe device. This means includes a wire 74 having wire lengths 90 and 92which runs vertically along capillary tube 14 from the bottom end ofstem portion 66 to vertical passage 27 in upper portion 22. Small hole33 is provided in upper portion 22 to allow wire 74 to pass external ofthe device and connect to the strain gauge arrangement 80 discussed inmore detail hereafter with reference to FIG. 2. At the diaphragm end,wire 74 is coiled at 76 to form a loop. The coil provides a substantiallength of the wire at that point. Coil 76 is adjacent to chamber 72where the volume of mercury which fills the device is great and issubjected to the highest temperatures. Thus, the application of heat tothe diaphragm end of stem portion 66 causes the greatest increase inresistance of wire 74 at 76. When the wire 74 including loop 76 isconnected in the bridge circuit of FIG. 3 in series with an arm of thebridge whose resistance decreases with increases in pressure, then theincrease in resistance of the loop 76 and the decrease in resistance ofthe bridge resistor, due to attendant heat caused by expansion of theliquid, tend to balance or compensate for one another.

In another embodiment of a temperature compensation means according tothe invention, the loop 76 is replaced by a short length of higherresistance wire, which is characterized by a larger resistance changefor a given temperature change than the rest of wire 76. This shortlength of higher resistance wire may be soldered directly between thewire lengths 90 and 92 and provides the same type of temperaturecompensation as loop 76.

Referring now to FIG. 2 there is shown a strain gauge arrangementaffixed to the top surface of cap 50 to detect the deflection ofdeformable'wall 53. This arrangement comprises windings 82 and 86 whoseresistance decreases with increases in pressure, and windings 84 and 88whose resistance increases with increases in pressure. Solder tabs 83,85, 87, 89, 91 and 93 are provided on the top surface of cap 50 so thatthe windings 82, 84, 86 and 88 can be connected externally as shown inFIG. 3. For example, temperature compensation wire 74 would be connectedbetween tabs 83 and 91. An input excitation voltage would be connectedacross tabs 85 and 89, and an output voltage would be sensed across tabs91 and 93.

FIG. 3 clearly illustrates the connections of the resistive windings ofthe strain gauge arrangement 80 shown in FIG. 2. The resistors 82A, 84A,86A and 88A schematically represent the windings 82, 84, 86 and 88,respectively. The resistors 90A and 92A schematically represent theresistance of the wire lengths 90 and 92 respectively of wire 74.Resistor 76A is the temperature compensation resistance andschematically depicts the wire loop 76 shown in FIG. 1.

An input signal would be applied across terminals 95, 97 and a pressureresponsive voltage would be measured across terminals 96, 98. Aspreviously mentioned, the temperature compensation arm includingresistors 90A, 92A and 76A is connected in series with resistance 82A,whose resistance decreases with increased pressure. This arrangementtends to maintain the strain gauge bridge in balance and independent oftemperature changes. The temperature compensation arm could also beconnected in series with resistance 86A, or alternatively separatetemperature compensation arms could be tied in series with resistors 82Aand 86A, respectively.

According to the present invention, cap 50 is preferably welded to upperportion 22. A weld joint is made between annular recesses 56 and 57 incap 50 and portion 22, respectively. When one desires to change pressureranges the cap 50 can be easily removed by machining or cutting and acap having a different thickness of wall 53 substituted therefore andrewelded.

Another feature of the present invention is concerned with high pressureranges. Screw threads 26 are provided on the outer surface of upperportion 22 so that a clamping device (not shown) can be threadedthereon. This clamping device would be somewhat U-shaped and would holdtightly against the outer portion of the top surface of cap 50. Thisclamping device would further aid in holding cap 50in place under highpressures.

In still another embodiment of the invention a flexible coupler maycomprise part of intermediate portion 34, preferably at its bottom end.Such a coupler is indicated at 46 in FIG. 1 of the above-cited Pastanpatent. Obviously such a coupler would have to be placed above portion60 and not between portion 60 and the screw threads 70 of stem portion22.

From the foregoing description those skilled in the art will appreciatethat numerous modifications may be made of this invention withoutdeparting from its spirit. Therefore, it is not intended to limit thebreadth of this invention to the embodiments illustrated and described.Rather, it is intended that the breadth of this invention be determinedby the appended claims.

What is claimed is:

l. A pressure transducer comprising,

an elongated frame having means defining a passage, a flat surface atone end thereof, and mating means adjacent the flat surface,

a capillary tube extending through the frame and terminating at one endadjacent another end of the frame,

a coupler closing the other end of the frame and defining with the framea chamber in communication with the capillary tube,

a liquid filled deformable sensor including a cap member having a recesstherein and mating means adjacent thereto for connecting with the matingmeans of the frame, said recess and flat surface defining a thindiscshaped compartment in communication with another end of thecapillary tube,

and a liquid filling the capillary tube, chamber and compartment forapplying the pressure exerted against the coupler to the sensor.

2. A pressure transducer according to claim 1 further characterized bysaid cap member defining a thin deflectable wall defining said recessand one side of said compartment, and said frame includes an upwardextending portion defining the flat surface and another side of saidcompartment.

3. A pressure transducer according to claim 1 further comprising straingage windings bonded to an outer surface of said deflectable wall tomeasure the pressure applied to the wall by the liquid.-

4. A pressure transducer according to claim 3 wherein four active straingauge windings are arranged in a bridge network.

5. A pressure transducer according to claim 1 further comprising atemperature compensating element positioned adjacent said coupler andadapted to be coupled to said sensor.

6. A pressure transducer according to claim 5 wherein said sensorcomprises a strain gauge bonded thereto including four active straingauge windings arranged in a bridge network with said temperaturecompensating element coupled to at least one of said windings.

7. A pressure transducer according to claim 6 wherein said temperaturecompensating element includes a winding coiled about the capillary tubeadjacent the coupler.

8. A pressure transducer according to claim 6 wherein said temperaturecompensating element includes a winding coupled in series with a windingof said strain gauge whose resistance decreases with increased pressure.

9. A pressure transducer according to claim 1 wherein said couplercomprises a soft flexible diaphragm.

10. A pressure transducer according to claim 1 wherein said liquid ismercury 11. A pressure transducer comprising;

a housing,

a capillary tube extending through the housing and terminating at oneend adjacent one end ofthe housing,

a coupler closing the one end of the housing and defining with thehousing a chamber in communication with the capillary tube,

saidhousing having means defining a sensor compartment near another endof the housing and in communication with another end of the capillarytube,

a liquid filling the capillary tube, chamber and compartment forapplying the pressure exerted against the coupler to the means defininga sensor compartment,

a temperature compensating element positioned adjacent said chamber,

and means coupling said element to said means defining a sensorcompartment,

said element compensating for temperature changes in the vicinity ofsaid coupler to alter pressures measured at said means defining a sensorcompartment,

wherein said temperature compensating element includes a winding of aplurality of turns coiled adjacent the coupler chamber.

12. A pressure transducer as set forth in claim 11 wherein said meansdefining a sensor compartment comprises a strain gauge arrangementincluding four strain gauge windings arranged in a bridge network withsaid temperature compensating element coupled to at least one of saidwindings.

13. A pressure transducer as set forth in claim 12 wherein said windingof said temperature compensating element is coupled in series with awinding of said strain gauge whose resistance decreases with increasedpressure.

14. A pressure transducer comprising;

a housing,

a capillary tube extending through the housing and terminating at oneend adjacent one end of the housing,

a coupler closing the one end of the housing and defining with thehousing a chamber in communication with the capillary tube,

said housing having means defining a sensor compartment near another endof the housing and in communication with another end of the capillarytube,

a liquid filling the capillary tube, chamber and compartment forapplying pressure exerted against the coupler to the means defining asensor compartment,

a temperature compensating length of wire disposed adjacent the one endof the capillary tube and the chamber,

and wire means coupling said length of wire to said means defining asensor compartment,

said length of wire compensating for temperature changes in the vicinityof said coupler to alter pressures measured at said means defining asensor compartment.

15. A pressure transducer as set forth in claim 14 further comprising astrain gauge arrangement including a plurality of strain gauge windingswith said length of wire coupled to at least one of said windings.

16. A pressure transducer as set forth in claim 15 wherein said lengthof wire couples in series with a winding of said strain gaugearrangement whose resistance decreases with increased pressure.

17. A pressure transducer as set forth in claim 14 wherein said lengthof wire is coiled about the one end of the capillary tube.

18. A pressure transducer as set forth in claim 14 wherein said lengthof wire has a difference per unit length resistance than the resistanceof said wire means.

1. A pressure transducer comprising, an elongated frame having meansdefining a passage, a flat surface at one end thereof, and mating meansadjacent the flat surface, a capillary tube extending through the frameand terminating at one end adjacent another end of the frame, a couplerclosing the other end of the frame and defining with the frame a chamberin communication with the capillary tube, a liquid filled deformablesensor including a cap member having a recess therein and mating meansadjacent thereto for connecting with the mating means of the frame, saidrecess and flat surface defining a thin disc-shaped compartment incommunication with another end of the capillary tube, and a liquidfilling the capillary tube, chamber and compartment for applying thepressure exerted against the coupler to the sensor.
 2. A pressuretransducer according to claim 1 further characterized by said cap memberdefining a thin deflectable wall defining said recess and one side ofsaid compartment, and said frame includes an upward extending portiondefining the flat surface and another side of said compartment.
 3. Apressure transducer according to claim 1 further comprising strain gagewindings bonded to an outer surface of said deflectable wall to measurethe pressure applied to the wall by the liquid.
 4. A pressure transduceraccording to claim 3 wherein four active strain gauge windings arearranged in a bridge network.
 5. A pressure transducer according toclaim 1 further comprising a temperature compensating element positionedadjacent said coupler and adapted to be coupled to said sensor.
 6. Apressure transducer according to claim 5 wherein said sensor comprises astrain gauge bonded thereto including four active strain gauge windingsarranged in a bridge network with said temperature compensating elementcoupled to at least one of said windings.
 7. A pressure transduceraccording to claim 6 wherein said temperature compensating elementincludes a winding coiled about the capillary tube adjacent the coupler.8. A pressure transducer according to claim 6 wherein said temperaturecompensating element includes a winding coupled in series with a windingof said strain gauge whose resistance decreases with increased pressure.9. A pressure transducer according to claim 1 wherein said couplercomprises a soft flexible diaphragm.
 10. A pressure transducer accordingto claim 1 wherein said liquid is mercury.
 11. A pressure transducercomprising; a housing, a capillary tube extending through the housingand terminating at one end adjacent one end of the housing, a couplerclosing the one end of the housing and defining with the housing achamber in communication with the capillary tube, said housing havingmeans defining a sensor compartment near another end of the housing andin communication with another end of the capillary tube, a liquidfilling the capillary tube, chamber and compartment for applying thepressure exerted against the coupler to the means defining a sensorcompartment, a temperature compensating element positioned adjacent saidchamber, and means coupling said element to said means defining a sensoRcompartment, said element compensating for temperature changes in thevicinity of said coupler to alter pressures measured at said meansdefining a sensor compartment, wherein said temperature compensatingelement includes a winding of a plurality of turns coiled adjacent thecoupler chamber.
 12. A pressure transducer as set forth in claim 11wherein said means defining a sensor compartment comprises a straingauge arrangement including four strain gauge windings arranged in abridge network with said temperature compensating element coupled to atleast one of said windings.
 13. A pressure transducer as set forth inclaim 12 wherein said winding of said temperature compensating elementis coupled in series with a winding of said strain gauge whoseresistance decreases with increased pressure.
 14. A pressure transducercomprising; a housing, a capillary tube extending through the housingand terminating at one end adjacent one end of the housing, a couplerclosing the one end of the housing and defining with the housing achamber in communication with the capillary tube, said housing havingmeans defining a sensor compartment near another end of the housing andin communication with another end of the capillary tube, a liquidfilling the capillary tube, chamber and compartment for applyingpressure exerted against the coupler to the means defining a sensorcompartment, a temperature compensating length of wire disposed adjacentthe one end of the capillary tube and the chamber, and wire meanscoupling said length of wire to said means defining a sensorcompartment, said length of wire compensating for temperature changes inthe vicinity of said coupler to alter pressures measured at said meansdefining a sensor compartment.
 15. A pressure transducer as set forth inclaim 14 further comprising a strain gauge arrangement including aplurality of strain gauge windings with said length of wire coupled toat least one of said windings.
 16. A pressure transducer as set forth inclaim 15 wherein said length of wire couples in series with a winding ofsaid strain gauge arrangement whose resistance decreases with increasedpressure.
 17. A pressure transducer as set forth in claim 14 whereinsaid length of wire is coiled about the one end of the capillary tube.18. A pressure transducer as set forth in claim 14 wherein said lengthof wire has a difference per unit length resistance than the resistanceof said wire means.